diff --git a/CHANGELOG.md b/CHANGELOG.md
index a366ef8..f1576c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,11 @@
 
 <!-- ## [4.2.0](https://github.com/sketch7/FluentlyHttpClient/compare/4.1.0...4.2.0) (2024-10-11) -->
 
+## [4.1.1](https://github.com/sketch7/FluentlyHttpClient/compare/4.1.0...4.1.1) (2025-07-24)
+
+### Features
+
+- **transform value:** format value per key
 
 ## [4.1.0](https://github.com/sketch7/FluentlyHttpClient/compare/4.0.0...4.1.0) (2024-10-11)
 
diff --git a/package.json b/package.json
index 1c79430..c789d2d 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@sketch7/fluently-http-client",
-  "version": "4.1.0",
+  "version": "4.1.1",
   "versionSuffix": "",
   "scripts": {
     "pack": "bash ./tools/pack.sh",
diff --git a/src/FluentlyHttpClient/Utils/QueryStringOptions.cs b/src/FluentlyHttpClient/Utils/QueryStringOptions.cs
index 6e92775..05d50f1 100644
--- a/src/FluentlyHttpClient/Utils/QueryStringOptions.cs
+++ b/src/FluentlyHttpClient/Utils/QueryStringOptions.cs
@@ -48,6 +48,7 @@ public record QueryStringOptions
 	/// Gets or sets the function to format a collection item. This will allow you to manipulate the value.
 	/// </summary>
 	public Func<object, string>? CollectionItemFormatter { get; set; }
+	public Dictionary<string, Func<object, string>>? CollectionValuePerKeyItemFormatter { get; set; }
 
 	/// <summary>
 	/// Gets or sets the function to format the key e.g. lowercase.
@@ -55,6 +56,7 @@ public record QueryStringOptions
 	internal Func<string, string> KeyFormatter { get; set; } = DefaultKeyFormatter;
 
 	internal Func<object, string>? ValueFormatter { get; set; }
+	internal Dictionary<string, Func<object, string>>? ValuePerKeyFormatter { get; set; }
 
 	internal Func<string, string> ValueEncoder { get; set; } = DefaultValueEncoder;
 
@@ -98,6 +100,18 @@ public QueryStringOptions WithValueFormatter(Func<object, string> configure)
 		return this;
 	}
 
+	/// <summary>
+	/// Gets or sets the function to format a value per key. This will allow you to manipulate the value by key
+	/// </summary>
+	/// <param name="configure"></param>
+	/// <returns></returns>
+	public QueryStringOptions WithValuePerKeyFormatter(Dictionary<string, Func<object, string>> configure)
+	{
+		ValuePerKeyFormatter = configure;
+		CollectionValuePerKeyItemFormatter = configure;
+		return this;
+	}
+
 	/// <summary>
 	/// Gets or sets the function to encode a value. This will allow you to encode the value. e.g. UrlEncode.
 	/// </summary>
diff --git a/src/FluentlyHttpClient/Utils/QueryStringUtils.cs b/src/FluentlyHttpClient/Utils/QueryStringUtils.cs
index 1fd3e6b..b5d463b 100644
--- a/src/FluentlyHttpClient/Utils/QueryStringUtils.cs
+++ b/src/FluentlyHttpClient/Utils/QueryStringUtils.cs
@@ -35,7 +35,7 @@ public static string ToQueryString<TKey, TValue>(this IDictionary<TKey, TValue>?
 
 			if (item.Value is string)
 			{
-				qs = AddQueryString(key, FormatValue(item.Value), qs, options.ValueEncoder);
+				qs = AddQueryString(key, FormatValue(key, item.Value), qs, options.ValueEncoder);
 				continue;
 			}
 
@@ -45,15 +45,23 @@ public static string ToQueryString<TKey, TValue>(this IDictionary<TKey, TValue>?
 					qs = BuildCollectionQueryString(key, values, qs, options);
 					break;
 				default:
-					qs = AddQueryString(key, FormatValue(item.Value), qs, options.ValueEncoder);
+					qs = AddQueryString(key, FormatValue(key, item.Value), qs, options.ValueEncoder);
 					break;
 			}
 		}
 
 		return qs;
 
-		string FormatValue(object value)
-			=> options.ValueFormatter == null ? value.ToString()! : options.ValueFormatter(value);
+		string FormatValue(object param, object value)
+		{
+			if (options.ValueFormatter != null)
+				return options.ValueFormatter(value);
+
+			if (options.ValuePerKeyFormatter != null && param is string && options.ValuePerKeyFormatter.TryGetValue((param as string)!, out var transform))
+				return transform(value);
+
+			return value.ToString()!;
+		}
 	}
 
 	/// <summary>
@@ -76,13 +84,18 @@ private static string BuildCollectionQueryString(string key, IEnumerable values,
 			? key
 			: options.CollectionKeyFormatter(key);
 
+		var collectionFormatter = options.CollectionItemFormatter;
+
+		if (collectionFormatter == null && options.CollectionValuePerKeyItemFormatter?.TryGetValue(key, out var formatter) is true)
+			collectionFormatter = formatter;
+
 		switch (options.CollectionMode)
 		{
 			case QueryStringCollectionMode.KeyPerValue:
 				foreach (var value in values)
 				{
-					var valueStr = options.CollectionItemFormatter != null
-						? options.CollectionItemFormatter(value)
+					var valueStr = collectionFormatter != null
+						? collectionFormatter(value)
 						: value.ToString();
 					qs = AddQueryString(key, valueStr, qs, options.ValueEncoder);
 				}
@@ -91,8 +104,8 @@ private static string BuildCollectionQueryString(string key, IEnumerable values,
 				var index = 0;
 				foreach (var value in values)
 				{
-					var valueStr = options.CollectionItemFormatter != null
-						? options.CollectionItemFormatter(value)
+					var valueStr = collectionFormatter != null
+						? collectionFormatter(value)
 						: value.ToString();
 					if (index == 0)
 						qs = AddQueryString(key, valueStr, qs, options.ValueEncoder);
diff --git a/test/FluentHttpRequestBuilderTest.cs b/test/FluentHttpRequestBuilderTest.cs
index e8c2b6c..5e5f40a 100644
--- a/test/FluentHttpRequestBuilderTest.cs
+++ b/test/FluentHttpRequestBuilderTest.cs
@@ -286,13 +286,40 @@ public void WithQueryParamsOptions_MultipleCallsShouldKeepConfiguring()
 		var request = builder.WithUri("/org/sketch7/heroes")
 			.WithQueryParamsOptions(opts => opts.CollectionMode = QueryStringCollectionMode.CommaSeparated)
 			.WithQueryParamsOptions(opts => opts.WithKeyFormatter(s => s.ToUpper()))
+			.WithQueryParamsOptions(opts => opts.WithValuePerKeyFormatter(new Dictionary<string, Func<object, string>>()
+			{
+				["POWERS"] = val => (val  is string valStr) ? valStr.ToUpper() : null!
+			}))
 			.WithQueryParams(new
 			{
 				Roles = new List<string> { "warrior", "assassin" },
+				Powers = new List<string> { "medium", "high" }
 			}
 			).Build();
 
-		Assert.Equal("/org/sketch7/heroes?ROLES=warrior,assassin", request.Uri.ToString());
+		Assert.Equal("/org/sketch7/heroes?ROLES=warrior,assassin&POWERS=MEDIUM,HIGH", request.Uri.ToString());
+	}
+
+	[Fact]
+	public void WithQueryParamsOptions_ValuePerKeyTransform()
+	{
+		var builder = GetNewRequestBuilder();
+		var request = builder.WithUri("/org/sketch7/heroes")
+			.WithQueryParamsOptions(opts => opts.WithValuePerKeyFormatter(new Dictionary<string, Func<object, string>>()
+			{
+				["power"] = val => (val is string valStr) ? valStr.ToUpper() : null!,
+				["dateTime"] = val => (val is DateTime valStr) ? valStr.ToString("yyyy-MM-dd'T'HH:mm:ss'Z'") : null!
+			}))
+			.WithQueryParamsOptions(opts => opts.WithValueEncoder(x => x))
+			.WithQueryParams(new
+			{
+				Role = "warrior",
+				Power = "medium",
+				DateTime = new DateTime(2025, 1, 1)
+			}
+			).Build();
+
+		Assert.Equal("/org/sketch7/heroes?role=warrior&power=MEDIUM&dateTime=2025-01-01T00:00:00Z", request.Uri.ToString());
 	}
 }